1. Field of the Invention
The present invention relates to the field of radiofrequency transmission and, more specifically, to the reception of a communication channel in a multiple-channel transmission system.
A specific example of application of the present invention is the field of radiotelephony which uses several communication channels in an allotted frequency band. For example, a so-called GSM (Global System Mobile) protocol uses a 25-MHz band centered on the 947.5 MHz frequency. Another example is the DCS (Digital Cellular System) protocol which uses a 25-MHz band centered on the 1.895-GHz frequency. In these two radiotelephony protocols, the 25-MHz band is divided into channels of 200 kHz assigned to the different communications. In such a system, each mobile phone, that is, each transmitter-receiver, is capable of using any of the transmission channels to complete a call.
The present invention more specifically relates to the reception of information which, in such systems or the like, has to be performed by extracting from the entire frequency band only those frequencies of the considered channel which are desired to be received, hereafter called the useful channel.
2. Discussion of the Related Art
The useful channel recovery, on the receive side, uses filtering techniques for suppressing any frequency located outside the frequency band of the useful channel.
Among receivers known to perform this function, one of the most current ones is the so-called heterodyne receiver. Such a receiver includes, on the path of the signal from a reception antenna, a first surface wave filter for rejecting any frequency outside the so-called useful reception band (Rx) assigned to the communication system (for example, all frequencies outside the xc2x112.5 MHz range around the 947.5 MHz frequency for the GSM), followed by a low noise amplifier for amplifying the signal, then by a mixer for performing a first frequency conversion, that is, a frequency shift to an intermediary frequency on the order of 100 MHz. The frequency of the local oscillator of the mixer is set so that an intermediary 100-MHz frequency corresponds to the central frequency of the useful channel. The use of such a mixer requires an image filter for removing a parasitic frequency band, or image band, due to the frequency shift. The image filter is placed upstream of the mixer and is used to push back the image band which is otherwise superposed to the useful band (here, the band centered on the intermediary frequency). Downstream of the mixer, a third, so-called intermediary band-pass filter is used, centered on the intermediary frequency to reduce useful band Rx. This band-pass filter has, for example, a bandwidth on the order of a few MHz centered on a central 100-MHz frequency. The signal then obtained is the signal resulting from a first frequency conversion which has to undergo a second frequency conversion to shift the intermediary frequency towards a zero frequency, to only recover the original baseband modulation.
The modulations most often used in transmission systems, in particular in radiotelephony systems, require a phase information. The second frequency conversion thus has to be performed in phase and in phase quadrature, to obtain a compound signal. This second frequency conversion is, in a heterodyne receiver, performed by two mixers of the signal resulting from the first frequency conversion with a frequency equal to the first intermediary frequency, that is, 100 MHz in the above a example. The 100-MHz frequency provided to the two mixers comes from a local oscillator and the mixers receive this local oscillation frequency respectively in phase and out of phase by 90xc2x0 to obtain, at the output of the second conversion, signals in phase and in phase quadrature. At the output of the second conversion, the baseband useful channel and a parasitic band centered on 200 MHz, which is easily suppressed with a low-pass filter, are obtained.
The signals resulting from the second frequency conversion are most often converted into digital signals by analog-to-digital converters, generally so-called delta-sigma converters, after which they are digitally demodulated.
More recently, a so-called direct conversion technique has been provided for radiofrequency transmission receivers.
FIG. 1 very schematically shows a conventional example of a direct conversion system of a radiofrequency signal converter.
As for a heterodyne receiver, the signals received from an antenna 1 are routed through a surface wave filter 2 having the function of rejecting the frequencies out of the reception band Rx of the considered system. Referring to the GSM example, this filter rejects the frequencies outside the useful band included between 935 MHz and 960 MHz. Filter 2 is followed, as previously, by a low noise amplifier (LNA) 3. The output of amplifier 3 is, in the direct conversion system illustrated by FIG. 1, sent to two mixers 4, 5, the respective inputs of which receive a frequency corresponding to the central frequency of the considered reception channel. This frequency is provided by a local oscillator 6 (LO) and mixers 4 and 5 respectively receive the central frequency of the channel in phase and out of phase by 90xc2x0so that their respective outputs are in phase and in phase quadrature. The outputs of mixers 4 and 5 thus directly provide the received signal in baseband since the frequency of local oscillator 6 is chosen for the intermediary frequency of the direct conversion to be null. The parasitic band is very far (centered on twice the frequency of local oscillator 6) from the useful band and can thus easily be suppressed by filtering. Each mixer 4, 5 is followed by a low-pass filter (LPF), respectively 7, 8, for suppressing the channels neighboring the channel which is desired to be received and the image band. Each filter 7, 8 is associated at its output with an automatic gain control amplifier (AGC), respectively 9, 10, the respective outputs of which form the outputs of the so-called null intermediary frequency direct conversion system.
Then, for example, analog-to-digital converters (CAN) 11, 12 followed by a digital demodulation circuit 13 are provided.
An advantage of a null intermediary frequency direct conversion system such as illustrated in FIG. 1 is that it is more easily integrable than a heterodyne receiver, since the two filters (image and intermediary filters) of a heterodyne receiver are not necessary therein. Currently, these filters are not integrable. A direct conversion receiver is thus generally less expensive than a heterodyne receiver. Further, its power consumption is lower and it generates less power dissipation.
However, a direct conversion receiver has the disadvantage that the useful channel is corrupted by noise coming, in particular, from the D.C. offset resulting from the superposition of the useful signal to the D.C. system supply signal. The D.C. offset is variable and thus has a frequency spectrum of low frequencies.
It should be noted that the D.C. offset of course exists in the case of a heterodyne receiver. However, since the useful channel is centered on a 100-MHz frequency, it does not superpose to this D.C. offset which is suppressed by the intermediary filter.
Another disadvantage of a null intermediary frequency direct conversion system is the low frequency noise, called the 1/f noise, which superposes on the converted signal.
A third known solution (not shown) for receiving a radiofrequency signal is to directly perform an analog-to-digital conversion of the received signal and to transfer the filtering to the digital side. Although such a solution can be implemented for relatively low frequencies (under one MHz), it can be difficult to apply to higher frequencies and, in particular, to radiotelephony systems having frequencies on the order of one GHz or above, due to the required sampling frequencies which are then too high for existing digital systems.
The International Patent Application 9708842 discloses a method for receiving radio frequency signals through an iterative process for extracting possible phase errors and gain errors. A drawback of this method is that the error detection must be permanently active, even during the reception of useful data. Another drawback is that this method is efficient only within a very narrow frequency band around the central frequency of the channel and is therefore not applicable to broad band systems.
European Patent Application 0501740 also discloses a system using an iterative method.
The present invention aims at providing a novel frequency conversion architecture which overcomes the disadvantages of conventional solutions. In particular, the present invention aims at overcoming the problem of the D.C. offset and of low frequency noise.
The present invention also aims at providing a solution which improves or optimizes the possibility of system integration and which, especially, maintains the advantages of a direct conversion system with respect to a heterodyne system.
The present invention also aims at providing a novel radiofrequency signal reception architecture using digital filtering. In particular, the present invention aims at providing a novel solution which, on the digital side, is particularly adapted to the frequency conversion at low intermediary frequency.
To achieve these and other objects, the present invention provides a frequency conversion receiver at low intermediary frequency including:
a first analog mixer of a received signal with a signal coming from a local oscillator at a first conversion frequency, the output of the first mixer defining a first path meant to be subject to an analog-to-digital conversion;
a second analog mixer of the incoming signal with the signal coming from the local oscillator out of phase by 90xc2x0, the output of the second mixer defining a second path meant to be subject to an analog-to-digital conversion; and
on the digital side, means for detecting a possible phase difference and a possible gain difference between the signals of the two paths, the first conversion frequency corresponding to the central frequency of the received channel, plus half the channel frequency band.
According to an embodiment of the present invention, the receiver includes, on the digital side, a first means for extracting a first measured value corresponding to the amplitude of the first path out of phase by 90xc2x0 and minus the amplitude of the second path, a second means for extracting a second value corresponding to the amplitude of the first path out of phase by 90xc2x0 and plus the amplitude of the second path, and means for measuring the respective amplitudes of the two paths.
According to an embodiment of the present invention, the receiver includes, on the digital side, means for applying, to at least one of the paths, a compensation based on the previously calculated values.
The present invention also provides a method of reception of radiofrequency signals of the type including a first analog frequency conversion performed on the received signal and providing two analog paths respectively in phase and in phase quadrature, and a second frequency conversion performed after quantization of signals resulting from the first frequency conversion, the method including, on the digital side, performing a compensation of possible phase and/or gain drifts between the two analog paths.
According to an embodiment of the present invention, the compensation is performed in a non-iterative manner.
According to an embodiment of the present invention, the following are extracted from the two digital paths: the amplitude of each path; a first measurement value corresponding to the amplitude of a first path out of phase by 90xc2x0 and minus the amplitude of the second path; and a second measurement value corresponding to the amplitude of the first path out of phase by 90xc2x0 and plus the amplitude of the second path.
According to an embodiment of the present invention, the median frequency of the useful band of the received channel is chosen to be the intermediary frequency of the useful band of the received channel.
According to an embodiment of the present invention, the second frequency conversion is performed at a null intermediary frequency.
The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.